The present invention relates to a method and to an apparatus for extruding a thermoplastic plastics material foam. More particularly, the present invention relates to a method for extruding a thermoplastic plastics material foam formed from a thermoplastic plastics material and a pore regulating substance in which these materials are fused, mixed and homogenized in an extrusion device and into which a propellant is injected into the fused mixture. The present invention also relates to an extrusion device suitable for carrying out such a method.
Extruded foam products have been known for a long time In the manufacture of such products, a nucleating agent must be employed.
Combustible propellants were initially used in the manufacture of such products. An early breakthrough in the production of such foams was the use of chlorofluorohydrocarbons (CFC) as the propellants instead of the combustible propellants. Pentane had also been used as propellant. However, it has recently been ascertained that CFC gases substantially accelerate the destruction of the ozone layer in the upper atmosphere of the earth. Accordingly, there has recently been a return to the use of pentane. In view of the flammability of pentane, use of alternative propellants is desirable.
In general, the extrusion devices which are to be used to produce foams do not differ substantially from conventional extrusion devices. However, additional costly safety measures, such as use of metering systems for the addition of the propellant, intensive ventilation and means for removing electrostatic charge from the semi-finished foam articles and products, must normally be used.